feat(check): add `--check-js` flag (#32235)
It was not possible to typecheck javascript files with deno without
modifying source code by either doing one of:
- create deno.json with compiler flag `checkJs: true`
- adding `// @ts-check` at the source file.
This makes quick typechecking awkward for javscript-only project that
doesn't use package.json (e.g https://github.com/simpelplant/charasort-bluearchive)
Made using gpt-5.3 codex high
---------
Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>
